Overload relays are electrical switches typically employed in industrial settings to protect electrical equipment from damage due to overheating in turn caused by excessive current flow. In a typical case, the electrical equipment is a three-phase motor which is connected to a power source through another relay commonly referred to as a contactor. A typical contactor is a heavy duty relay having three switched power paths for making and breaking each of the circuits connected to the three phase power source. The motion required to make and break the contacts is provided magnetically as a result of current flow through a coil which in turn is energized by a current whose flow is controlled by another switch, typically remotely located.
In a conventional setup, an overload relay is connected in series with the control switch for the coil of the contactor. When an overload condition is detected by the overload relay, the same cuts off power to the coil of the contactor, allowing the contactor to open and disconnect the electrical equipment that is controlled by the contactor from the source of power to prevent injury to the electrical equipment.
In the past, overload relays have utilized resistive heaters for each phase which are in heat transfer relation with a bimetallic element which in turn controls a switch. When an overload is sensed as, for example, when there is sufficient heat input from the resistive heater to the bi-metallic element, the bimetallic element opens its associated switch to de-energize the contactor coil and disconnect the associated piece of electrical equipment from the source of power.
More recently, the resistive heater-bi-metallic element type of relay has been supplanted by electronic overload relays. See, for example, commonly assigned U.S. Pat. No. 5,179,495 issued Jan. 12, 1993, to Zuzuly, the entire disclosure of which is herein incorporated by reference. Outputs of such circuitry typically are relatively low powered and as a consequence, in order for the output to control the contactor coil current, a solid state switch may be required. The solid state switch may, in turn, control flow to a relatively low power contact mechanism which in turn is operable to control the flow of current to the contactor as well as to operate an indicator. In the usual case, the indicator will be a light which will be illuminated upon the occurrence of a disconnect resulting from an overload. One such contact mechanism is disclosed in my commonly assigned co-pending application entitled, "Trip Mechanism for an Overload Relay", Serial No. 08/838,904, Filed Apr. 11, 1997, the entire disclosure of which is herein incorporated by reference.
The trip mechanism therein disclosed uses so-called "bridging" contacts which is to say, an elongated contact bar is brought into contact with two spaced, fixed contacts as a result of movement of the armature. The contact bar is biased against a cross member on a post carried by the armature and which provides a fulcrum for the contact bar. Because the armature pivots, the contact bars are moved in an arcuate path as the armature shifts between two bistable positions and, of course, the contact bar may pivot somewhat on the fulcrum as well. Consequently, there is the possibility that one end of the contact bar will contact one of the fixed contacts before the other end of the contact bar contacts its associated fixed contact. Desirably, however, the contact bar should contact both fixed contacts simultaneously.
Additionally, there is concern for environmental grime being deposited on the contact surfaces of one or more of the contacts. Particularly when the electric circuit being made or broken by the contacts is of a relatively low power, such grime can interfere with the generation of a clean signal upon the closing of the contacts. Thus, the trip mechanism of my co-pending application provides for a measure of wiping of the fixed contacts by the moveable contacts when they closed to avoid the effects of such environmental grime. At the same time, it is desirable to provide substantial wiping wherein the wiping movement of the moveable contacts on the fixed contacts is a positively driven movement to assure that the desired wiping action will take place.
The present invention is directed to overcoming one or more of the above.